1. Field of the Invention
This invention is in the field of optical source and in particular, in the field of high optical power broadband optical sources free of facet reflections over an extended bandwidth.
2. Description of the Related Arts
Optical sources and in particular high power optical sources having a broad spectral width, are key components in improving performance of many applications such as, OCT (optical coherence tomography), IFOG (interferometric fiber optic gyroscope), high-power illumination, optical sensing, optical spectroscopy, to name just a few. Often times, semiconductor optical sources are preferred for such applications for their low drive current, efficiency and small footprint. A conventional broadband semiconductor light emitting device for example, a Light Emitting Diode (LED) provides very low output power and is not suitable for many of these applications. Alternative edge emitting semiconductor light-generating and amplifying devices, such as Super Luminescent Diodes (SLD) provide higher power output but have a smaller bandwidth. Although these devices may be operated at a high gain, often times spectral quality and in particular, spectral modulation puts a practical limit on the output power.
Most semiconductor optical sources comprise a waveguide structure extending between two parallel end facets, and include a p-n double hetero junction constructed from multiple layers of semiconductor materials of appropriate thickness and composition, on a semiconducting substrate. Light from such an edge emitting device propagates at one or both end facets depending upon the reflectivities of the two end facets. For a waveguide structure to provide a high output power, it needs to be operated at a drive current high enough to produce optical gain in the waveguide medium. However, operating the device at high gain also reduces the bandwidth.
In the past, different waveguide designs have been explored for achieving higher output power from a waveguide light source. In a non-patent literature publication entitled “Low spectral modulation high-power output from a new AlGaAs Superluminescent diode/optical amplifier structure”, authored by Alphonse et al. and published in Applied Physics Letters vol. 55 (22), November 1989, pp. 2289-2291, a light source having a tilted waveguide structure is disclosed to exhibit higher output power. A similar design for a SLD is disclosed in the U.S. Pat. No. 4,958,355 issued on Sep. 18, 1999 to Alphonse et al. A tilted waveguide design for this device allows operation at a high gain level without lasing by reducing reflectivity at the end facets of the waveguide. The resulting source provides about 20 mW (milli Watt) of output power, and a spectral modulation of about 1%. While this device may be adequate for certain applications, spectral modulation may still be a limiting factor for applications requiring significantly higher power.
In a different design disclosed in the U.S. Pat. No. 6,339,606 issued on Jan. 15, 2002, to Alphonse, a diamond waveguide structure including multiple tapered sections is disclosed (FIGS. 6 & 7). In this structure the waveguide extended between the two end facets is expanded in the center to achieve higher output power. By placing the waveguide at an angle with respect to one or both end facets, higher output power and low spectral modulation is expected from such a device. However, when operated at high drive current to achieve high output power, the facet reflectivity still contributes significant amount of spectral modulation and reduction in the bandwidth, thereby limiting the utility of such source as a broadband source.
Edge emitting semiconductor optical sources having even high output power and enhanced bandwidth are disclosed in the U.S. Pat. Nos. 8,269,977 and 8,259,304 issued on Sep. 18, 2012, and Sep. 4, 2012, respectively, both to Alphonse. The disclosures of the above mentioned patents are hereby incorporated by reference, in their entirety. More specifically, the optical sources described therein comprise a Semiconductor Optical Amplifier (SOA) and Super Luminescent Diodes (SLD) having a waveguide, and in particular a ridge waveguide disposed between two etched channels at a small angle with respect to end facets of the guiding region.
One important design aspect of said broadband sources is to provide different optical coatings having predetermined reflectivity profile for example, antireflection, raised-edge reflectivity, etc. at the two end facets, resulting in high optical power and extended bandwidth that surpass emission from a SOA or a SLD alone. However, spectral profile of said optical sources exhibits a discrete set of emission lines resembling teeth of a comb. While this type of source is beneficial for certain applications (e.g. OCT), it may not be useful for other applications where a broad spectral profile source having high output power and low spectral modulation is preferable (e.g. IFOG).